Web-reliant apps always needed to get their data from servers, sometimes multiple servers, and apps always needed to choose a data format.

When apps choose a distinct format and permission their servers, they are not interoperable. When apps choose the same format and open their servers, they are interoperable.

This was true before Nostr, and is true for all web app paradigms.

Relays are just websites doubling as open web hosts. The issue there is all the good relays are gonna censor you eventually, just like any popular website.